How do we know where we are? How do we get from one place to another? To successfully navigate, humans and animals must accurately encode external cues in the surrounding environment. For humans, environmental cues are predominantly visual. For example, to get to the post office, you might turn right at the grocery store, and then head towards the tall bakery sign adjacent to the post office. Now imagine traveling to the post office in pure darkness. What cues would you use? You could travel towards the “hustle and bustle” of town, then follow the scent of cooling pies. In this hypothetical, you would have to rely on auditory and olfactory cues, which typically form “gradients.” Gradient cues, such as scent, sound, luminance, and geomagnetic fields, emanate from a source and produce multiple vectors that provide directional information, facilitating spatial orientation (scent: Wallraff, 2004; sound: King & Parsons, 1999; luminance: Petie, Garm, & Nilsson, 2011; & magnetic fields: Boles & Lohmann, 2003).

Cues such as auditory or olfactory gradients are encoded by a single sensory modality. The slope of the terrain underfoot is a unique gradient because it is a multi-modal cue; slope can be perceived by the kinesthetic, vestibular, and/or visual sensory systems (Nardi, Newcombe, & Shipley, 2011, 2012). For human adults and many animal species (e.g., rats: Miniaci, Scotto, & Bures, 1999; pigeons: Nardi, Nitsch, & Bingman, 2010), slope is highly salient and successfully guides navigation, although human adults exhibit significant variability in performance (Nardi et al., 2011, 2012). In a series of studies, Nardi and colleagues (2011, 2012) asked human adults to locate a target, hidden in one of four corners, in a completely featureless, but sloped square enclosure. Following disorientation (mild spinning in a swivel chair), only the slope accurately predicted a goal’s location. Performance in this task was significantly above chance, suggesting that slope was a salient and reliable cue for humans, with one caveat: men consistently outperformed women (Nardi et al., 2011) and were significantly more confident in their target choice (Nardi et al., 2012). Men noticed the slope, and effectively used this cue, significantly more than women; 1.4 SDs more to be exact (Nardi et al., 2011), an effect size exceeding those detected in tests of mental rotation (Voyer, Voyer, & Bryden, 1995).

Does female footwear (e.g., heels) impede slope perception? When given flat slippers, women’s performance remained the same (Nardi et al., 2011), and no correlation was detected between reported mean heel height and performance (Nardi et al., 2012). To test the “footwear hypothesis” more concretely, and to examine the developmental origins of slope perception and use in general, we tested the use of slope in school-age children (8-10 years).

Participants were assigned to one of three conditions: control (no slope, featureless room); slope (5° slope, otherwise featureless room); and ball drop (identical to slope condition, with the addition of a “ball drop” demonstration). In each condition, participants explored the enclosure and were asked if they noticed “anything unusual about the room.” Positive slope recognition was recorded if the sloped floor was verbally noted using such words as “slanted, tilted, sloped, uneven, uphill, downhill, higher end, lower end” and so forth. Immediately afterward, participants were disoriented and asked to recall the location of a hidden target over four trials. Because the room was square and completely featureless, successful target identification indicated the use of slope for reorientation. (see Figure 1)

Figure 1. Views of the enclosure, digital and actual.

Data analyses indicate that in the presence of slope, children are significantly more likely to locate the target, although males significantly outperform females. Furthermore, it appears that the ability to notice the slope (termed “slope recognition”), not sex per se, may be driving this gap in performance. Males’ and females’ performance, analyzed by both response accuracy and error type, did not significantly differ when both sexes noticed, or did not notice, the slope of the room.

Although males were significantly more likely to notice the slope in both experimental conditions, performance did not significantly differ by sex when slope recognition was held constant. Additionally, performance in the ball drop condition was significantly greater for participants who noticed the slope unprompted, compared to those who only noticed the slope following the “ball drop” demonstration. Thus, performance may be influenced by one’s ability to notice the slope on one’s own. (see Figures 2 and 3)

So, what factors may lead to greater slope salience for males? There could be a variety of reasons, but let’s concentrate here on environmental exposure to slope. A wealth of research shows that boys and girls are differentially socialized, specifically by play type (Miller, 1987). Boys are given a wider variety of toys, ranging from blocks to cars to sports equipment, while girls are given toys largely related to domestic activities, such as dollhouses and mock cooking ware (Baenninger & Newcombe, 1989; Bradbard, 1985). Masculine toys elicit movement, and often incorporate sloped terrains (e.g., sports are played on non-uniform fields, toy cars come with multi-level ramps, etc.), potentially sharpening the kinesthetic, vestibular, and visual sensory systems’ ability to perceive slope (Liss, 1983).

Previous experience with sloped terrains might hone the ability to perceive slope at varying degrees. Compared to a novice, an expert has multiple conceptual categories within a single domain, and is thus better equipped to discriminate small variations among stimuli (e.g., x-rays: Sowden, Davies, & Roling, 2000; gender: Biederman & Shiffrar, 1987; phonemes: Liberman, Harris, Hoffman, & Griffith, 1957; etc.). Termed “categorical perception,” this theory posits that people are better able to perceive differences among stimuli when the stimuli belong to different conceptual categories (Goldstein & Hendrickson, 2010; Goldstone, 1998). Applied to the current study, minimal exposure with sloped terrains would limit the number of slope conceptual categories. Thus, girls would be less likely to differentiate a 5-degree incline from a flat surface, as their perceptual systems would be less tuned to this mild differentiation in terrain. Furthermore, when the slope was made explicitly salient, participants who did not initially notice the slope did not use the slope simply because they had little experience in doing so. Because the slope lacked reliability as a navigational cue, it appeared irrelevant to the task at hand and was thus ranked behind other erroneous, unstable cues. Future work is needed to examine slope discrimination in similar paradigms across other sensory modalities, to better define the factors that shape slope perception in the visual, kinesthetic, and/or vestibular sensory systems.